docs: add community group QR code

[ningzimu]

Summary

• add a community group section to the Chinese and English READMEs
• add the community QR image under assets
• allow the QR image through the repository image ignore rules

Verification

• git diff --check

[ningzimu]

Reviewed. This PR only updates README.md / README_en.md, adds the community QR image asset, and adjusts .gitignore to allow that specific asset.

Verification performed:

• inspected PR file patches via GitHub API
• confirmed PR is mergeable
• checked CI: changelog-check and pr-title are passing
• verified assets/codex-ppt-community-qr.png is a valid 1200x1200 PNG
• ran git diff --check against base commit: passed

No issues found from my side.